The present invention relates to a zoom lens system for use in a compact camera with a lens shutter and, more particularly, to a compact zoom lens system in which the distance from the first lens surface to the image-forming plane is relatively short.
In the field of compact still cameras with lens shutters, a bifocal-type compact camera which is available for both wide-angle photography and telephotography as recently been introduced. The bifocal-type compact camera is provided either with two interchangeable lens systems or with a lens system having a conversion lens component to be moved into and out of the optical train.
Against this background, the demand for a compact camera with a zoom lens system as the taking lens system has recently increased.
There are already well known a number of kinds of zoom lens systems, which, however, are for use in single lens reflex cameras. Therefore, these zoom lens systems must have a back focal distance longer than 35 mm in order to provide the necessary space in the single lens reflex camera for allowing a so-called quick-return mirror to swing up and down; and so the length from the first lens surface to the image forming or focal plane is relatively long. For these reasons, a zoom lens system is disadvantageous when the camera must be as small as possible. Therefore, a compact camera equipped with a conventional zoom lens system would not be convenient to carry. Convenience of carrying is one of the most important advantages of a compact camera and should by no means be disregarded.
In order to overcome the difficulty with which zoom lens systems are attended, various zoom lens systems have been proposed. Among these is a zoom lens system disclosed in Japanese Patent Kokai (Unexamined Publication) No. 56-128,911, which includes a first positive refractive power lens group and a second negative refractive power lens group which are adapted to move relatively in order to vary the distance therebetween, thereby to vary the focal length thereof for zooming. This zoom lens system, while being sufficiently short in overall length when adjusted for wide-angle photography, is limited to a zooming ratio of approximately 1.5, which is poor in comparison with that of a bifocal-type compact camera.
Another zoom lens system, disclosed in Japanese Patent Kokai (Unexamined Publication) No. 58-184,916, comprises a first and a second positive refraction power lens group, and a third negative refraction power lens group, and has a zooming ratio comparable to that of a bifocal-type compact camera, a sufficiently shortened overall length and a desirable optical performance. However, this zoom lens system is not practical, because of its very short back focal distance of a little over 1 mm for wide-angle photography. Additionally, since the zoom lens system has a constant aperture ratio of F/4.5 over the whole range of zooming, it is necessary to vary the size of the aperture according to the focal length. This requirement makes it difficult or even impossible to incorporate the zoom lens system in a compact camera with aperture-shutter means. If it is assumed that the size of the aperture is maintained constant over the whole range of zooming, the aperture ratio will be approximately F/8.7 at the telephotographic extremity of the zooming range, which is to slow in lens speed for the zoom lens system to be incorporated in still cameras.
In the zoom lens system disclosed in the last-mentioned Japanese Patent Kokai, it is necessary to move axially aperture means simultaneously with the axial movement of the lens groups in order to focus. Due to the necessity of moving the aperture means, it is very difficult to incorporate the zoom lens system in an automatic focusing camera.
In the zoom lens systems described above which comprise two lens groups, if there is a demand for magnification, say, 2X, it will become very difficult to correct the chromatic aberration over the whole range of zooming because of the concave lens component involved in the rear lens group, which acts differently on both longitudinal and lateral chromatic aberrations.